Interpretive Summary: Artemisia annua L. (annual wormwood, Asteraceae) and its secondary metabolite artemisinin, a unique sesquiterpene lactone with an endoperoxide bridge, has gained much attention due to its antimalarial properties. Artemisinin is a complex structure that is requires a significant amount of energy for the plant to synthesize. So, what are the benefits to A. annua of producing this unique structure? And what is the ecological role of artemisinin? This review addresses these questions, discussing evidence of the potential utility of artemisinin in protecting the plant from insects and other herbivores, as wells as pathogens and competing plant species. Abiotic factors affecting the artemisinin production, as well as mechanisms of artemisinin release to the surroundings are also discussed, and new data are provided on the toxicity of artemisinin towards soil and aquatic organisms and fungi. The antifungal and antibacterial effects reported are not very pronounced. Several studies have reported that extracts of A. annua have insecticidal effects, though few studies have proven that artemisinin could be the single compound responsible for the observed effects. However, the pathogen(s) or insect(s) that provided the selection pressure for the evolution of artemisinin synthesis may not have been represented in the research thus far conducted. The well-established high level of selective phytotoxicity of artemisinin under realistic test conditions indicates that plant/plant allelopathy is at least one beneficial function of artemisinin to the producing plant. The different release routes from A. annua to the surrounding environment support this rationale for allelopathy, although more research into the ecological role of this important compound is needed.

Technical Abstract:
Artemisia annua L. (annual wormwood, Asteraceae) and its secondary metabolite artemisinin, a unique sesquiterpene lactone with an endoperoxide bridge, has gained much attention due to its antimalarial properties. Artemisinin is a complex structure that is requires a significant amount of energy for the plant to synthesize. So, what are the benefits to A. annua of producing this unique structure? And what is the ecological role of artemisinin? This review addresses these questions, discussing evidence of the potential utility of artemisinin in protecting the plant from insects and other herbivores, as wells as pathogens and competing plant species. Abiotic factors affecting the artemisinin production, as well as mechanisms of artemisinin release to the surroundings are also discussed, and new data are provided on the toxicity of artemisinin towards soil and aquatic organisms and fungi. The antifungal and antibacterial effects reported are not very pronounced. Several studies have reported that extracts of A. annua have insecticidal effects, though few studies have proven that artemisinin could be the single compound responsible for the observed effects. However, the pathogen(s) or insect(s) that provided the selection pressure for the evolution of artemisinin synthesis may not have been represented in the research thus far conducted. The well-established high level of selective phytotoxicity of artemisinin under realistic test conditions indicates that plant/plant allelopathy is at least one beneficial function of artemisinin to the producing plant. The different release routes from A. annua to the surrounding environment support this rationale for allelopathy, although more research into the ecological role of this important compound is needed.